Most of the devices featured in this technical guide use the magnetic attraction
or repulsion system of operation. This rugged electro-mechanical design has
proven itself for more than 60 years and millions of heavy duty
applications. The electro-mechanical system consists of an attractor of magnetic
material, usually 430 stainless steel, or a magnet inside the control and
a movable magnet outside the device. As the attractor moves in or out of
the magnetic field the magnet moves toward or away from the switch assembly to
either make or break one or more electrical contacts. This switch action performs
the task of operating equipment, alarms, shutdowns, and/or
reports such functions to a computer or central location. Controls featuring
this magnet operated system can be
used at temperatures up to 800°F (425°C) and pressures up to 1750 psig
(121 bar). Liquid level controls are available to handle fluids from water
to
more aggressive liquids requiring all 316SS
construction. Dry bulk controls can handle materials from carbon black to soybean
flour. Electrical contacts of SPST, SPDT, or DPDT configuration are provided
to accommodate applications from 12A AC, 10A DC, to
milliampere requirements of high technology low current, low voltage intrinsically
safe or "dry" circuits. General
purpose, weatherproof or explosion-proof enclosures protect the electrical switch
assemblies from the environment.

Dwyer Dry Bulk or Liquid Level Controls are designed to perform two functions:

To maintain a level. This task is usually completed by using a control with
a wide deadband, or two fixed deadband controls and a holding relay. Thus
when the level of a liquid or dry bulk material reaches a
pre-determined high (or low) point a pump is shut off or turned on. This cycle
is repeated continuously such as in a sump, tank, lubrication equipment, bin
or other pressurized or non-pressurized vessel.

To monitor a safe level or detect a not-to-exceed level - that is to act as an alarm, or shut down, or
both. This can be either high or low functions or both. This is accomplished by using a control with a fixed
deadband. There are a great variety of controls suitable for this purpose.

There are basically 8 determining factors when selecting a control for a specific
function.

If the control is to be externally mounted, can the chamber be a welded type such as for use with clean
non-corrosive liquids or should it be flanged so that the unit can be inspected and/or easily maintained? Welded
chamber benefits are that they are usually less expensive than flanged controls and can be used with higher
pressure and temperature. The disadvantage is that the unit cannot be disassembled for cleaning or inspection.
Flanged units can be disassembled but are more expensive and have lower pressure and temperature capability.

Controls for top mounted service are generally capable of performing a greater variety of functions such as
operating 1, 2 or 3 pumps, providing high and low alarm or combining functions such as operating a pump and
providing both high and low alarms in one unit.